Insulated gate bipolar transistors (IGBT) are three-terminal power semiconductor devices. The IGBT combines the simple gate-driven characteristics of a metal-oxide-semiconductor field effect transistor (MOSFET) with the high current, low saturation voltage capability of bipolar transistors. This is accomplished by combining an isolated gate FET, which serves as the control input of the IGBT, with a bipolar transistor, which serves as the switch for the IGBT, in a single device.
FIG. 1A is a cross-sectional schematic diagram illustrating a conventional IGBT according to the prior art. A conventional IGBT comprises a field stop layer 103 of n− type supported by a p+ substrate 101. An n− epitaxial/voltage blocking layer 105 is grown on top of the field stop layer 103. One or more cells can be formed with the epitaxial/voltage blocking layer 105. Each cell may include a p− type body region 107 formed within the epitaxial/voltage blocking layer 105 and one or more n+ emitter regions 109 formed within the p− type body region 107. Each cell may further include gate electrode insulator 111 (e.g., an oxide) formed on an exposed part of the p− type body region 107 and n+ emitter regions 109. A gate electrode 113 is formed on the gate insulator 111. Emitter electrodes 115 are formed on different portions of the body region 107 and emitter region 109. A collector electrode 117 can be formed on a back surface of the p+ substrate 101. The IGBT 100 is constructed similarly to an n-channel vertical MOSFET, except that the n+ drain is replaced with a p+ collector layer 101, thus forming a vertical PNP bipolar junction transistor. The additional p+ collector layer 101 creates a cascade connection of a PNP bipolar junction transistor with the surface n-channel MOSFET.
The IGBT provides more optimal performance in certain applications over a conventional MOSFET device. This is mainly due to the fact that the IGBT exhibits a significantly lower forward voltage drop in comparison to the MOSFET. However, the increase in forward voltage drop is off-set by the slow switching speeds of the IGBT device. Minority carriers that are injected into the n− epitaxial/voltage blocking layer 105 take time to enter and exit or recombine at turn on and turn off, resulting in longer switching times and higher switching losses in comparison to the MOSFET.
In response to the slow switching speeds of conventional IGBT devices, anode-shorted IGBT devices have been developed. The anode-shorted IGBT is superior to the conventional IGBT in that it exhibits more desirable switching characteristics while retaining the improved forward voltage drop. FIG. 1B is a cross-sectional schematic diagram illustrating a conventional anode-shorted IGBT in accordance with the prior art. The anode-shorted IGBT 100′ shown in FIG. 1B is essentially the same as the IGBT in FIG. 1A, except that the p+ substrate 101 in FIG. 1A is replaced by a layer consisting of alternating p− type regions 101 and n− type regions 119 in FIG. 1B. By alternating p− regions 101 and n− type regions 119 the IGBT is effectively provided an additional body diode as well as improved switching speeds.
Embodiments of the present invention relate to methods for fabricating such an anode-shorted insulated gate bipolar transistor (IGBT).